1. Field of the Invention
The present invention relates generally to an optical distance measuring apparatus for measuring a distance between an object to be measured (target) and a distance measuring operation location where a distance measuring operation is performed with the apparatus. More particularly, the present invention relates to a two-eyed type optical distance measuring apparatus of the aforementioned kind operable in accordance with a phase difference detecting process.
2. Description of the Prior Art
To facilitate understanding of the present invention, a typical conventional two-eyed type optical distance measuring apparatus of the aforementioned kind will briefly be described below with reference to FIG. 2.
FIG. 2 is an illustrative view of the conventional two-eyed type optical distance measuring apparatus which schematically shows the structure of the same. Specifically, the apparatus includes a housing a in which a light source b and a light receiving element c are arranged. A light beam (emitted light beam L.sub.1) output from the light source b in the form of a modulated distance measuring light beam is modulated in a light emitting circuit (not shown) and is emitted through a emitted light lens d toward a light reflecting member P which is constructed of a corner prism placed at a distance measuring location, i.e., a target.
A reflected light beam L.sub.2 reflected from the light reflecting member P is once collected by a light receiving lens e and then received by the light receiving element c. On the other hand, a part of the modulated distance measuring light beam output from the light source b is introduced into the light receiving element c through a reference light passage including a first reflective mirror f and a second reflective mirror g, as a reference light beam L.sub.3 so that the reference light beam L.sub.3 is received by the light receiving element c. Subsequently, the reference light beam L.sub.3 is subjected to photoelectrical conversion in a light receiving circuit (not shown) in which a phase difference between the optical phase of the reference light beam L.sub.3 and the optical phase of the reflected light beam L.sub.2 is detected so as to determine a distance between the apparatus and the target by performing a comparative calculating operation with the aid of a calculating circuit (not shown).
A problem of the conventional apparatus constructed as mentioned above, is that a distance measuring operation is restrictively performed only within the range of 1 to 2 km with the apparatus including a corner prism having a high reflectivity as a light reflecting member. In recent years, however, earnest requests have been raised from users for providing a two-eyed type optical distance measuring apparatus which assures that a longer distance can reliably be measured therewith. When the longer distance is practically measured, it is absolutely necessary that an output from the light source b is increased and a sensitivity of the light receiving element c is substantially elevated.
However, when the output from the light source b is increased to satisfy the foregoing necessity, a part of the reflected light beam L.sub.2 is unavoidably received by the light receiving element c as a stray light via the lens surfaces of the light emitting lens d and the light receiving lens e separately arranged on the front surface of the housing a in parallel with each other or via the reference light passage. Thus, the resulting distance measurement is erroneously measured. Thus, there is a possibility that a measuring accuracy is undesirably degraded.
On the contrary, when the sensitivity of the light receiving element c is substantially elevated, a phenomenon of electrical induction is liable to appear, because a light emitting portion (i.e., the light source b) and a light receiving portion (i.e., the light receiving element c) are arranged in a single mirror sleeve of the housing a. With the conventional apparatus, however, it is very difficult to completely prevent appearance of the phenomenon of electrical induction as mentioned above.
In addition, in a case where a shorter distance is measured, an optical quantity of the reflected light beam received by the light receiving element c is remarkably increased, causing an output from the light receiving element c to be undesirably saturated. As a result, a distance measuring operation is erroneously performed which has reduced accuracy. To obviate this problem, it is necessary to variably control an optical quantity of the emitted light beam.